32 GENESIS AND DEVELOPMENT 



closed side toward the wind, and I must subscribe to his opinion, as J 

 have not, on any of the dunes visited my me, been able to observe a 

 phenomenon of that kind. 



Summing up the foregoing discussion on dune forms, we would say 

 that: 



1. Dunes are formed in lines transverse to wind in unobstructed 



places. 



2. Dunes are formed in lines parallel with wind in places where 



some kind of obstruction is in the path of the wind. 



3. Wandering dunes have their convexity in the direction of the 



prevailing wind. 



POLYGENETIC ORIGIN OF A DUNE COMPLEX. 



The fundamental principles of dune formation established in the 

 previous section must be recognized at the' outset of this discussion in 

 order to facilitate an understanding of the conditions which cause the 

 origin and development of a dune complex. With this term, dune 

 complex or dune tract, as distinguished from a solitary dune or a dune 

 massiv, we signify a collection of secondary dune hills interspersed with 

 deep gullies. 



The dune complex is formed behind the chain of primary dunes, 

 when the rate of travel of the sand is locally diminished without a 

 corresponding decrease in the supply of sand. The formation takes 

 place either through older dunes having been broken up into rough 

 hummocks by the wind, or through secondary embryonic dunes being 

 started by the piling up of drifting sand around some obstacle, in most 

 cases a congregation of plants. 



One of the most important factors determining the development of 

 a dune complex is the presence of ground moisture. Without this factor 

 we would not be able to explain the apparent irregularity of formation 

 of a group of small dunes, where the quality of sand ought to have called 

 for a regular dune massiv. The action of wind alone is insufficient for 

 this modelling of the surface, but in connection with the greater co- 

 herence of the sand particles caused by the ground moisture the 

 phenomenon is easily explicable. 



It is the irregularity in action of wind, caused by the breaking up 

 of the regular even surface of a dune or a sand field into small elevations 



